Dockerfile - set ENV to result of command

Question

Is it possible to set a docker ENV variable to the result of a command? Like:

ENV MY_VAR whoami

i want MY_VAR to get the value \"root\" or

Answer 1

As an addition to @DarkSideF's answer, if you want to reuse the result of a previous command in your Dockerfile during in the build process, you can use the following workaround:

1. run a command, store the result in a file
2. use command substitution to get the previous result from that file into another command

For example :

RUN echo "bla" > ./result
RUN echo $(cat ./result)

For something cleaner, you can use also the following gist which provides a small CLI called envstore.py :

RUN envstore.py set MY_VAR bla
RUN echo $(envstore.py get MY_VAR)

Or you can use python-dotenv library which has a similar CLI.